Thermal retracting actuator

ABSTRACT

A thermal actuator for a rotating shaft shutdown seal that has a piston with a portion of its axial length enclosed within a chamber shell with a material that expands upon a rise in temperature. The portion of the actual length of the piston within the chamber has at least two different diameters with the larger diameter leading in the direction of travel of the piston. Upon a rise in temperature, expansion of the material surrounding the piston within the chamber creates a force on the piston in the desired direction of travel. Below a preselected temperature the piston is positively locked with a passive release when the preselected temperature is reached.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. patent applicationSer. Nos. 13/970,899, filed Aug. 20, 2013, which application is acontinuation-in-part of parent application Ser. No. 13/798,632, filedMar. 13, 2013, entitled “Pump Seal With Thermal Retracting Actuator,”and claims priority to U.S. Provisional Patent Application Ser. No.61/862,304, filed Aug. 5, 2013, entitled “Reactor Coolant Pump Shut DownSeal Thermal Retracting Actuator With Thermal Safety Lock.”

BACKGROUND

1. Field

This invention pertains generally to rotary shaft seals and, moreparticularly to a thermally actuated seal for a centrifugal pump and inparticular to a new thermal actuator for such a seal.

2. Related Art

In pressurized water nuclear power plants a reactor coolant system isused to transport heat from the reactor core to steam generators for theproduction of steam. The steam is then used to drive a turbine generatorfor the production of useful work. The reactor coolant system includes aplurality of separate cooling loops, each connected to the reactor coreand containing a steam generator and a reactor coolant pump.

The reactor coolant pump typically is a vertical, single stage,centrifugal pump designed to move large volumes of reactor coolant athigh temperatures and pressures, for example, 550° F. (280° C.) and atpressures of approximately 2,250 psia (155 bar). The pump basicallyincludes three general sections from bottom to top; hydraulic, shaftseal and motor sections. The lower hydraulic section includes animpeller mounted on a lower end of the pump shaft which is operablewithin the pump casing to pump reactor coolant about the respectiveloop. The upper motor section includes a motor which is coupled to drivethe pump shaft. The middle shaft seal section includes three tandem sealassemblies; lower primary (number 1 seal), middle secondary, and uppertertiary seal assemblies. The seal assemblies are located concentric to,and near the top end of, the pump shaft and their combined purpose is toprovide for minimal reactor coolant leakage along the pump shaft to thecontainment atmosphere during normal operating conditions.Representative examples of pump shaft seal assemblies known in the priorart are described in U.S. Pat. Nos. 3,522,948; 3,529,838; 3,632,117;3,720,222 and 4,275,891.

The pump shaft seal assemblies which mechanically seal the interfacebetween the stationary pump pressure boundary and the rotating shaft,must be capable of containing the high system pressure (approximately2,250 psi (155 bar)) without excessive leakage. The tandem arrangementof three seal assemblies is used to break down the pressure in stages.These three mechanical pump seal assemblies are controlled leakage sealswhich in operation, allow a minimal amount of controlled leakage at eachstage while preventing excessive leakage of the reactor coolant from theprimary coolant system to the respective seal leakoff ports.

The pump seal assemblies are normally maintained at temperatures wellbelow those of the primary coolant system, either through the injectionof cool fluid at the seal assemblies or through the use of a heatexchanger which cools the primary fluid before it reaches the sealassemblies. Theorized failure of these systems may expose the sealassemblies to high temperatures which will likely cause the controlledleakage of the seal assemblies to increase dramatically. When the causeof the loss of all nuclear fuel cooling in the reactor core is due tolosing all AC power, the seal leakoff has no means of returning to thecoolant system without electricity to power the makeup pumps. Controlledleakage without the means of makeup could hypothetically lead to reactorcoolant uncovering the reactor core and subsequent core damage.

Consequently, a need exists for an effective way to back up the standardseal assemblies in the event of a coincidental loss of all fuel coolingand loss of makeup pumping. Preferably, such a back up seal should beoperable upon loss of power or other cause for the loss of makeuppumping capacity to substantially seal the shaft from leakage.

SUMMARY

The foregoing objectives are achieved, in accordance with thisinvention, by a thermally actuated shutdown seal for a shaft of reducedspeed or stopped rotating equipment such as a pump, compressor or thelike, that is designed to restrict the normal leakage of coolant througha shaft seal. The shutdown seal claimed hereafter is useful for sealingany equipment having a narrow flow annulus between its shaft andhousing.

The shutdown seal is characterized by a “split ring” that is designed(i) to surround the shaft with an annulus therebetween during normaloperation and (ii) to constrict against the shaft when the shaft slowsbelow a predetermined speed or stops rotating. The split ring hasconfronting ends that are maintained in spaced relationship by a spacerwhen the shaft is rotating during normal online operation. When theshaft slows or stops rotating and the temperature in the housing rises,the spacer is removed from the confronting ends of the split ring andthe split ring constricts against the shaft as the confronting ends ofthe split ring approach each other, which blocks a substantial portionof the leakage of coolant through the annulus.

Preferably, the shutdown seal also has a pliable polymer seal ring whichis urged against the shaft by an increase in pressure in the housingwhen the split ring blocks the leakage of coolant through the annulus.

In particular, this invention provides such a seal with an improvedactuator for removing the spacer from between the confronting ends ofthe split ring when the liquid in the annulus rises above a preselectedtemperature so the split ring can constrict to narrow or substantiallyseal the portion of the annulus covered by the split ring. The actuatorincludes a cylinder having an axial dimension with a piston axiallymoveable within the cylinder with the cylinder having an upper and lowerend which is sealed around the piston. A piston rod is connected at oneend to the piston and at another end to the spacer. A cavity occupies aspace within the cylinder between the upper and lower ends, throughwhich space the piston travels. An axial dimension of the piston extendsthrough the space within the cavity when the spacer is disposed betweenthe confronting ends of the split ring. The axial dimension of thepiston has at least two separate diameters with a largest of thediameters leading a smaller of the diameters in a direction of travel ofthe piston to remove the spacer from the confronting ends of the splitring. A material occupies at least a portion of the space within thecavity. The material expands upon an increase in temperature to exert aforce on the piston that causes the piston to move in a direction toremove the spacer from between the confronting ends when the materialrises above a preselected temperature. Preferably, the force is exertedover an area around a circumference of the piston wherein at least aportion of the at least two diameters of the piston extend.

In one embodiment, the actuator includes a first seal supported betweenthe cavity and the piston at a lower end of the cavity and a second sealsupported between the cavity and the piston at an upper end of thecavity with the first and second seals being operable to confine thematerial to the cavity. Preferably, the first and second seals are cupseals and are constructed of PEEK. In this embodiment, the actuator mayalso include backup seals for either or both of the first and secondseals. Preferably, the backup seals are O-ring seals and desirably, theO-ring seals are formed from EPDM or HNBR. In another embodiment, thesupport for the first seal or a support for the second seal is designedto relieve a pressure within the cavity when the pressure exceeds apredetermined value and, desirably, the material is in thermalcommunication with the liquid.

In another embodiment the actuator includes a thermally activated safetylock configured to prevent the piston from moving in the cylinder in adirection that will remove the spacer from between the confronting endsof the split ring when the material is below the preselected temperatureand free the piston to move and remove the spacer from confronting endsof the split ring when the material rises above the preselectedtemperature. Preferably, the thermal safety lock is configured topassively unlock the piston when the fluid is above the preselectedtemperature. In one embodiment the thermally activated safety lockcomprises a pin that is suspended from one end of the cylinder andextends in a direction that the piston moves to remove the spacer fromthe confronting ends of the split ring. The pin extends at leastpartially within a recess in an end of the piston. A substantialremainder of the recess is substantially filled with a thermallyactivated material, wherein the thermally activated material has aviscosity at temperatures below the preselected temperature thatprevents the thermally activated material from flowing alongside a sideof the pin and out of the recess. At temperatures substantially at orabove the preselected temperature the thermally activated material has areduced viscosity that enables it to flow alongside the side of the pinand out of the recess. The resulting displacement of the thermallyactivated material enables the piston to move in a direction to removethe spacer from between the confronting ends of the split ring. Thethermally activated material may, for example be a polymer such aspolyethylene.

BRIEF DESCRIPTION OF THE DRAWINGS

A further understanding of the invention can be gained from thefollowing description of the preferred embodiments when read inconjunction with the accompanying drawings in which:

FIG. 1 is a schematic representation of one cooling loop of aconventional nuclear reactor cooling system which includes a steamgenerator and reactor coolant pump connected in series in a closed loopsystem with the reactor;

FIG. 2 is a cutaway perspective view of the shaft seal section of areactor coolant pump, illustrating in cross section the seal housing andthe lower primary, middle secondary, and upper tertiary seal assemblieswhich are disposed within the seal housing and surround the pump shaft;

FIG. 3 is an enlarged cross sectional view of a portion of the sealhousing and seal assemblies of the reactor coolant pump of FIG. 2;

FIG. 4 is a sectional view of the shaft seal arrangement showing anenlarged view of the lower primary seal shown in FIGS. 2 and 3, to whichthis invention may be applied;

FIG. 5 is an enlarged portion of the insert of a primary seal shown inFIG. 4 with a portion of the pump shaft and the shutdown seal of thisinvention hatched with the shutdown seal shown as employing a thermallyactuated mechanical piston to remove the spacer from the split ring;

FIG. 6 is an enlarged view of the piston arrangement shown schematicallyin FIG. 5 with the piston in the fully extended position with the spacerinserted between the opposing ends of the split ring of the shutdownseal that can benefit from this invention;

FIG. 7 is a sectional view that shows the piston arrangement of FIG. 8employed by the prior art showing the piston in a state before anactuation event in which the spacer is removed from between the opposingends of the split ring;

FIG. 8 is a sectional view of an improved actuation mechanism inaccordance with this invention which can be applied to remove the spacerof the shutdown seal shown in FIG. 7;

FIG. 9 is a sectional view that shows the piston arrangement of a secondembodiment of this invention;

FIG. 10 is a cross-sectional view of the embodiment shown in FIG. 9taken along the lines A-A thereof;

FIG. 11 is an enlarged portion of the insert of the primary sealincorporating the shutdown seal embodiment illustrated in FIGS. 9 and10;

FIG. 12 is a sectional view that shows the piston arrangement of a thirdembodiment of this invention; and

FIG. 13 is a sectional view of an alternate embodiment for locking thepiston prior to thermal actuation of the shutdown mechanism.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following description, like reference characters designate likeor corresponding parts throughout the several view. Also, in thefollowing description, it should be understood that such terms ofdirection as “forward,” “rearward,” “left,” “right,” “upwardly,”“downwardly,” and the like, are words of convenience and are not to beconstrued as limiting terms.

Prior Art Reactor Cooling Pump

To understand the invention, it is helpful to understand one environmentin which the invention will operate. However, it should be appreciatedthat the invention has many other applications. Referring to FIG. 1,there is shown a schematic representation of one of a plurality ofreactor coolant loops 10 of a conventional nuclear reactor coolantsystem. The coolant loop 10 includes a steam generator 12 and reactorcoolant pump 14 connected in series in a closed loop coolant system withthe nuclear reactor 16. The steam generator 12 includes primary heatexchange tubes 18 communicating with inlet and outlet plenums 20, 22 ofthe steam generator 12. The inlet plenum 20 of the steam generator 12 isconnected in flow communication with the outlet of the reactor core 16for receiving hot coolant therefrom along flow path 24, commonlyreferred to as the hot leg of the closed loop system. The outlet plenum22 of the steam generator 12 is connected in flow communication with aninlet section side of the reactor coolant pump 14 along flow paths 26 ofthe closed loop system. The outlet pressure side of the reactor coolantpump 14 is connected in flow communication with the inlet of the reactorcore 16 for feeding relatively cold coolant thereto along flow path 28of the cold leg of the closed loop system.

The coolant pump 14 pumps the coolant under high pressure about theclosed loop system. Particularly, hot coolant emanating from the reactor16 is conducted to the inlet plenum 20 of the steam generator 12 andthrough the heat exchange tubes 18 in communication therewith. While inthe heat exchange tubes 18, the hot coolant flows in heat exchangerelationship with cool feedwater supplied to the steam generator 12 viaa conventional means (not shown). The feedwater is heated and portionsthereof is changed to steam for use in driving a turbine generator (notshown). The coolant, whose temperature has been reduced by the heatexchange, is then recirculated to the reactor 16 via the coolant pump14.

The reactor coolant pump 14 must be capable of moving large volumes ofreactor coolant at high temperatures and pressures about the closed loopsystem. Although, the temperature of the coolant flowing from the steamgenerator 12 through the pump 14 as a result of the heat exchange hasbeen cooled substantially below the temperature of the coolant flowingto the steam generator 12 from the reactor 16 before heat exchange, itstemperature is still relatively high being typically about 550° F. (288°C.). To maintain the coolant in a liquid state at these relatively hightemperatures, the system is pressurized by injection pumps (not shown)and operates at pressures that are approximately 2,250 psia (155 bar).

As seen in FIGS. 2 and 3, the prior art reactor coolant pump 14generally includes a pump housing 30 which terminates at one end in aseal housing 32. The pump also includes a pump shaft 34 extendingcentrally of the pump housing 30 and being sealed and rotatably mountedwithin the seal housing 32. Although not shown, the bottom portion ofthe pump shaft 34 is connected to an impeller, while a top portionthereof is connected to a high horsepower, induction type electricmotor. When the motor rotates the shaft 34, the impeller within theinterior 36 of the pump housing 30 causes the pressurized reactorcoolant to flow through the reactor coolant system. This pressurizedcoolant applies an upwardly directed hydrostatic load upon the shaft 34since the outer portion of the seal housing 32 is surrounded by theambient atmosphere.

In order that the pump shaft 34 might rotate freely within the sealhousing 32 while maintaining the 2,250 psia (155 bar) pressure boundarybetween the pump housing interior 36 and the outside of the seal housing32, tandemly arranged lower primary, middle secondary and upper tertiaryseal assemblies 38, 40, 42 are provided in the positions illustrated inFIGS. 2 and 3 about the pump shaft 34 within the seal housing 32. Thelower primary seal 38 which performs most of the pressure sealing(approximately 2,200 psi (152 bar)) is of the noncontacting hydrostatictype, whereas the middle secondary and upper tertiary seal assemblies40, 42 are of the contacting or rubbing mechanical type.

Each of the seal assemblies 38, 40, 42 of the pump 14 generally includesa respective annular runner 44, 46, 48 which is mounted to the pumpshaft 34 for rotation therewith and a respective annular seal ring 50,52, 54 which is stationarally mounted within the seal housing 32. Therespective runners 44, 46, 48 and the seal rings 50, 52, 54 have top andbottom surfaces 56, 58, 60 and 62, 64, 66 which face one another. Thefacing surfaces 56, 62 of the runner 44 and seal ring 50 of the lowerprimary sealing assembly 38 normally do not contact one another butinstead a film of fluid normally flows between them. On the other hand,the face surfaces 58, 64 and 60, 66 of the runners and seal rings 46, 52and 48, 54 of the middle secondary and upper tertiary seal assemblies 40and 42 normally contact or rub against one another.

Because the primary sealing assembly 38 normally operates in afilm-riding mode, some provision must be made for handling cooling fluidwhich “leaks off” in the annular space between the seal housing 32 andthe shaft 34 rotatably mounted thereto. Accordingly, the seal housing 32includes a primary leakoff port 69, whereas leakoff ports 71 accommodatecoolant fluid leakoff from the secondary and tertiary seal assemblies40, 42.

FIG. 4 is a cross section of the seal housing in the area of the number1 or primary lower seal of the type illustrated in FIGS. 2 and 3 andprovides a better understanding of the operation of the number 1 sealand how it will interface with this invention. The structure showncomprises a housing 32 having annular wall 33 adapted to form a pressurechamber 35 within the housing 32; a shaft 34 rotatably mounted withinthe housing 32; a seal runner assembly 44 and a seal ring assembly 50disposed within the housing 32. As previously mentioned, the shaft 34may be driven by a suitable motor (not shown) and utilized to drive theimpeller of a centrifugal pump (not shown) which circulates the coolantin the pressurized system. Injection water may be supplied to thechamber 35 at a higher pressure than that developed by the pump. Therunner assembly 44 comprises an annular holder 70 and an annular sealplate 72. Similarly, the seal ring assembly 50 comprises a holder 74 andan annular face plate 76.

The holder 70 rotates with the shaft 34 since it is mounted on anannular support 78 which engages a shoulder 80 on the shaft 34 and issecured to the shaft by means of a sleeve 82 which is assembled onto theshaft 34 between the shaft and an upwardly extending leg 84 of thesupport 78 which is generally L-shaped in cross section. It should beappreciated that although this embodiment of the invention is beingdescribed as applied to a pump that employs a sleeve over the pumpshaft, the invention can be employed equally as well on pump shafts thatdo not employ sleeves. A shoulder 86 on the holder 70 rests on the upperend of the leg 84, and a shoulder 88 on the sleeve 82 retains the holder70 on the support 84. A pin 90 is pressed into a recess 92 in the sleeve82 and engages an axial slot 94 in the holder 70. An axial clampingforce is exerted on the sleeve 82 and the support 78 from a nut (notshown) which causes the sleeve 82 and the support 78 to rotate with theshaft 34. The pin 90, in turn, causes the holder 70 to rotate with thesleeve 82 which rotates with the shaft 34. O-ring seals 96 and 98 areprovided between the support 78 and the shaft 34 and the holder 70,respectively. An O-ring seal 100 is also provided in the interface 102between the holder 70 and the face plate 72.

The face plate 72 is composed of a corrosion and erosion resistantmaterial having substantially the same coefficient of thermal expansionas the material of which the holder 70 is composed, and the holder 70has a high elastic modulus. Similarly, the face plate 76 is composed ofa corrosion and erosion resistant material having substantially the samecoefficient of thermal expansion as the material of the holder 74 whichhas a high elastic modulus. Examples of suitable materials are carbidesand ceramics. An O-ring seal 104 is provided in the interface 106between the holder 74 and the face plate 76.

The holder 74 is movably mounted on a downwardly extending leg 108 of anannular seal ring insert 110 which is generally L-shaped in crosssection. The insert 110 is retained in the housing 32 by cap screws 112.An O-ring seal 114 is provided in the interface between the insert 110and the housing 32. Similarly, O-ring seal 118 is provided in theinterface 120 between the holder 74 and the leg 108 of the insert 110.Rotative movement of the holder 74 is prevented by the pin 122 which ispressed into the insert 110. The pin 122 extends into a well 124 in theholder 74 with sufficient clearance between the wall of the well 126 andthe pin 122 to permit axial movement of the holder 74 but limit rotativemovement of the holder 74.

The face plate 76 is attached to the holder 74 by clamping means 128which includes a retainer ring 130, a clamp ring 132, a lock ring 134, aplurality of cap screws 136 and belleville springs 138 mounted on thecap screw 136 between the lock ring 134 and the clamp ring 132. The capscrews 136 extend through the retainer ring 130, the clamp ring 132, thebelleville springs 138 and are threaded into the lock rings 134. Theinterface 106 of the holder 74 is recessed at 140 to provide an annularfulcrum 142 on the interface at an outside diameter which is less thanthe outside diameter of the interface of the face plate 76. The retainerring 130 has an inwardly extending flange with a ridge 144 which engagesthe portion 146 of the face plate 76 extending beyond the fulcrum 142.The clamp ring 132 has an inwardly extending flange with a ridge 148which engages a face plate 150 on the holder 74. Thus, when the capscrews 136 are tightened to draw the clamp 132 and the retainer ring 130towards each other, a force is produced which exerts a cantilever effecton the face plate 76 about the fulcrum 142. During the clamping action,the belleville springs 138 are partly compressed and the face plate 76is deformed by the clamping force.

The face plate 72 is attached to the holder 70 by a clamping means 151in a manner similar to that described with reference to the face plate76. However, the fulcrum 152 on the interface 102 of the holder 70 islocated closer to the outside diameter of the face plate 72 than is thefulcrum 142 on the holder 74. Thus, the clamping force on the face plate72 does not produce as much deformation of the face plate about thefulcrum 152 as is produced on the face plate 76. If desired, thefulcrums 142 and 152 may be placed at the same locations with respect totheir corresponding face plates.

As previously described, the seal ring 50 is mounted for limited axialmovement relative the shaft 34 and the seal runner assembly 44. Also,rotative movement of the seal ring assembly 50 is limited by theanti-rotational pin 122 which fits loosely in the well 124 in the sealring holder 74. A seal face 154 on the face plate 76 is biased towardthe confronting seal face 156 on the face plate 72 by gravity.

In operation of the pump driven by the shaft 34, surfaces 158 and 160 ofthe seal ring holder 174 are subjected to the full pressure in the highpressure chamber 35. It is desirable to provide a pressure barrierbetween the high pressure chamber 35 and an annular low pressure region162 adjacent the sleeve 82. The seal ring assembly is utilized as thepressure barrier means, but permits a controlled amount of fluid leakageflow to the region 162 from the pressure chamber 35 through a seal gap164 provided between the confronting seal surfaces 154 and 156 on theseal plate 76 and 72, respectively.

During operation, a balanced or equilibrium position of the axiallymoveable seal ring assembly 50 is maintained in accordance with thepressure on opposing faces of the seal ring assembly. The thickness ofthe fluid in the gap 164 and, consequently, the amount of leakage flowthrough the gap 164 is determined by the configuration of the gap 164.

In order to obtain a self-restoration of the relative position of theseal ring assembly 50 and the runner assembly 44 upon a variation in theseal gap 164, a fluid flow path of decreasing thickness is provided froma high pressure edge or extremity 166 to a position between the sealfaced extremities. More specifically, in the structure illustrated, thefluid flow path of decreasing thickness extends between the outer edge166 and an intermediate concentric circle located at 168 on the sealingface 154.

As shown in the present structure, the decreasing flow path thickness isformed by tapering the surface 154 slightly away from the confrontingsurface 156 of the face plate 72 between the circle 168 and the outeredge 166 of the face plate 76. The angle between the surfaces 154 and156 shown in the drawing is exaggerated. This configuration or structureis known as a tapered-face seal. The operation of a seal of this type isfully described in U.S. Pat. No. 3,347,552, issued Oct. 17, 1967 toErling Frisch.

The current shutdown seal is fully described in U.S. Pat. 8,356,972,issued Jan. 22, 2013 and assigned to the Assignee of this invention. Theshutdown seal, described in that patent is illustrated in FIGS. 5-7 andprovides an additional seal 170 in the pump 14 as a backup safety orshutdown device which is actuatable to prevent excessive leakage alongthe shaft 34 between it and the seal assemblies 38, 40, 42 of the pumpin the event of a loss of seal cooling. As shown in FIG. 5, the shutdownseal 170 is situated in a machined groove in the annular opening in theinsert 110 of the primary number 1 seal 38. The shutdown seal ischaracterized by a “split ring” 172 that is designed (i) to surround theshaft 34 with an annulus 174 therebetween during normal operation and(ii) to constrict against the shaft 34 when the shaft significantlyslows or stops rotating after a loss of seal cooling. The split ring 172is a single piece discontinuous ring member that is split axially andthe confronting ends are maintained in a spaced relationship by a spacer176 during normal pump operation. In FIG. 5, the opposing ends of thesplit ring 172 are machined in a tongue-and-groove configuration so thatthe tongue can ride in the groove as the ends of the split ring overlap.In another embodiment, the opposing ends may be butt ended or have amitered half lap joint so the ends overlap. The spacer 176 is shown inthe gap to keep the opposing ends of the split ring 172 from closing onthe shaft 34 to maintain the annulus 174 opened for controlled leakageduring operation. In accordance with the embodiment illustrated in FIG.5, the shutdown seal is activated when the temperature of the seal risesas a result of a loss of cooling and preferably rotation as the pumpshaft is slowed or stopped. The spacer is responsive to the rise intemperature (either because the shaft has significantly slowed orstopped rotating or for any other reason) to be removed from theconfronting ends of the split ring 172. This causes the confronting endsof the split ring to constrict against the shaft 34 as the confrontingends of the split ring approach each other, which blocks the leakage ofcoolant through the flow annulus 174. Preferably, the split ring andshaft (or shaft sleeve where a sleeve is employed over the shaft) areconstructed from gall resistant materials, so that if actuated on arotating shaft gall balls will not be created which would otherwiseserve as a wedge to open a leak path between the sealing surfaces.Materials such as 17-4 stainless for both the split ring and the shafthave proven to work well. A pliable polymer seal ring 178 is preferablysituated around the shaft 34 against the split ring 172 between thesplit ring and a solid retaining seat ring 180. The pliable polymer sealring 178 is urged against the shaft by an increase in pressure in thehousing when the split ring restricts the leakage of coolant through theannulus 174, thus forming a tight seal.

FIG. 5 schematically depicts a shutdown seal 170 of the type describedabove installed in the reactor coolant pump of FIG. 4. The shutdown sealof FIG. 5 is designed to activate after a loss of seal cooling when thepump shaft 34 slows or is not rotating. The shutdown seal is locatedwithin the pump housing, encircling the shaft 34. In the case of thetype of reactor coolant pump illustrated in FIGS. 2-4, the number 1 sealinsert may be modified to accommodate the shutdown seal by machining outa portion of the inner diameter at the top flange. Until activated, theshutdown seal 170 is substantially completely contained within the spaceonce taken up by the number 1 insert prior to modification, thussubstantially unaltering the annulus 174 between it and the shaft 34. Inthis way, coolant flow through the annulus 174 along the shaft 34 is notsubstantially impeded during normal operation of the rotating equipment.

FIG. 5 shows a shutdown seal 170 made up of a retractable spacer 176holding the confronting ends of the split ring 172 open. The retractablespacer 172 is activated by a thermally responsive mechanical device 184,such as the piston 186 described hereafter with regard to FIG. 6. Whenthe spacer 176 is retracted from the ends of the split ring 172, thesplit ring 172 snaps shut, constricting around the shaft 34, while alsoremaining retained in the modified number 1 seal insert 110. The splitring 172 sits on a wave spring 182 that forces the split ring 172 upagainst the seal 178 which pushes against the retaining ring 180. Thepressure drop caused by the interruption of the flow through the annulus174 also forces the split 172 and seal ring 178 upwards, ensuring atight seal between all of the sealing surfaces. The split ring 172 sitson a wave spring 182 that forces the split ring 172 up against theprimary sealing ring 178 to ensure an initial sealing contact so thepressure drop across the split ring 172 is also acting on the primarysealing ring 178.

FIGS. 6 and 7 depict the spacer 176 and actuator assembly 184 before anactuation event. The actuator 184 as shown in FIGS. 6 and 7, iscomprised of a canned piston 186 for restraining a spring loaded spacer176. Within the can is a wax 188 that changes phase at the desiredactivation temperature, e.g., 280° F. (138° C.) for reactor coolantpumps, as further explained herein. This change in phase results in asubstantial increase in volume of the wax 188. For example, a wax suchas Octacosane will increase about 17% in volume. When the wax 188changes phase and expands, it pushes a piston head 190 away from thepump shaft 34. When the piston head 190 moves, balls 192, that were onceheld in place by the piston 190, will drop out of the way and allow acompressed spring 194 to expand which pushes back the plunger 196 thatis connected to the spacer 176. As the spring 194 expands it pushes theplunger which pulls the spacer 176 with it, thus retracting the spacer176 from between the split ring ends.

Thus, thermal activation is achieved as follows: As temperature rises,the wax 188 changes state and expands. Two HNBR (Hydrogenated NitrileButadiene Rubber) O-ring seals 198 are used to contain the wax with theupper O-ring providing a sliding interface for the cam 190. Expansion ofthe wax translates cam 190 permitting ball bearings in the race 192 todisengage plunger 196 from the housing 200. With the ball bearingsdisengaged, compression spring 194 translates plunger 196 upward alongwith spacer 176 thus releasing the piston ring and activating theshutdown seal.

Improved Pump Shutdown Seal Actuator

FIG. 8 shows an improved thermal retracting actuator. As describedearlier, translation of the spacer 176 permits closure of a split ringenabling activation of the shutdown seal. When the temperature rises andreaches the phase transition point of the wax 188, the wax volume mayincrease up to approximately 17%. If the volume is held constant, thewax pressure will increase and can exceed 10,000 pounds per square inch(68,947.6 kpa). Piston 196 has a larger diameter D₁ and a smallerdiameter D₂ with corresponding cross sectional areas A₁ and A_(2.) Asthe pressure (P) increases, a translational force (F) is applied to thepiston 196 equal to the product of wax pressure and the difference incross sectional areas, i.e., F=P×(A₁−A₂). With such an arrangement shownin FIG. 8, a typical piston force may be in the range of 50 to 100pounds (22.7-45.4 kg) while achieving adequate piston travel to removethe spacer 176 from the split ring. This is a significant increase overthe approximate force of 15 pounds (6.8 kg) available from thecompression spring 194 shown in FIG. 7. Cup seals 204 and 206 provide apressure boundary for containment of the wax 188. They may beconstructed from PEEK (polyetheretherketone) and have sufficientstrength to contain the wax 188 at high pressure and are chemicallycompatible with both the wax and the surrounding reactor coolant. O-ringseals 208, 210 and 212 are made of EPDM (ethylene-propylene dieneM-class rubber) or HNBR which are compatible with the reactor coolant.In the event the PEEK seal should fail during activation, the EPDM orHNBR seal can act as a redundant pressure boundary. The EPDM seal canwithstand short term exposure to the wax. End cap 214 is free to slidewithin the housing 216 and is secured in place with multiple shear pins218. In the event that the piston 196 travels full stroke and the waxpressure continues to rise, pins 218 shear to release end cap 214permitting seal 206 to decouple from the housing 216 thereby releasingexcess wax volume and reducing the pressure to a safe state.

Since the entire retracting assembly 202 can be subjected to higher thanatmospheric pressure, several radial openings 220 are oriented about theupper flange of piston 196. Without the radial openings 220 it may bepossible that the head of the piston 196 could seal against the matingend cap 214. The external pressure (without radial openings present)could induce an undesirable axial force to the piston 196.

While it may not be required, sleeve 222 is placed over the exposeddiameter of piston 196 to maintain the piston free from contaminantswhich may be present in the surrounding environment. The sleeve may beconstructed of polypropylene which may melt when the activationtemperature is reached. Alternately, a small wiper may be placed in theend of housing 216 to remove unwanted debris during translation of thepiston.

FIG. 9 shows an alternative embodiment of f the invention with adifferent configuration. Alternative end cap 214 is secured inalternative housing with a spiral retaining ring 224. Spring 226provides a small force to maintain the piston 196 in the extendedposition prior to actuation.

Housing 216 contains at least two pockets 228 where the wall thicknessof the chamber containing the wax is reduced in thickness T1. In theevent that the piston 196 travels full stroke and the wax 188 pressurecontinues to rise, the housing wall can bulge at the pockets 228 therebyreleasing excess wax volume and reducing the pressure to a safe state.Section A-A of FIG. 9 is shown in FIG. 10 and illustrates across-section of the housing 216 at the location of the pockets 228. Thethinner wall (T1) can bulge when the wax pressure becomes excessive.Heavier wall (T2) helps maintain structural integrity of the housing216.

Another configuration is to have a housing where the thin wall section(T1) is continuous for 360°. Since the actuator will have performed itsfunction at the time when the wax pressure can become excessive,structural integrity from a thicker section (T2) is not necessary.

FIG. 11 shows a cross-section of the alternative retracting actuator asapplied to the insert of the primary seal shutdown seal.

While the foregoing embodiment has a spring to prevent inadvertentmovement of the piston, it is highly desirable to provide a more robustmechanism as inadvertent actuation would be extremely costly due toshutdown of the power plant. FIG. 12 illustrates such a robustmechanism.

FIG. 12 shows a third embodiment of the retracting actuator used in theshut down seal. Like reference characters are used among the severalfigures to designate corresponding components. Thermal activation isachieved when temperature rises causing the wax 188 to change state andexpand which gives rise to pressure, as explained above. As the waxpressure increases, a translational force is applied to the piston 196equal to the product of wax pressure and the difference in crosssectional areas of the piston. Without pin 232 in place, piston 196translates spacer 172 towards the housing 200. Removal of spacer 176permits closure of a split piston ring 172 enabling operation of thereactor coolant pump shaft shutdown seal.

A typical piston force may be in the range of 50 to 100 lbs whileachieving adequate piston travel to remove the spacer 176 from the splitring 172. O-ring 204 and cup seal 206 provide a pressure boundary forcontainment of the wax 188. O-ring seals 210 and 212 act as a redundantpressure boundary for the wax and provide isolation from the surroundingfluid from entering the wax chamber. Wiper 208 in conjunction with cupseal 206 are semi-rigid and act as dual bushings keeping piston 196centered within housing 200. The primary function of wiper 208 is toexclude foreign material from entering into the housing 200. The wiperis also a seal to minimize surrounding fluid from entering new O-ring204.

In one embodiment, in order to keep piston 196 from translation prior tothermal activation, plug 230 encapsulated in the piston 196 issufficiently rigid to hold metal pin 232 in place. As can be seen inFIG. 12, plug 230 has a thin wall at location 234 that provides aflexible boundary which allows for a slight press fit between theoutside diameter of pin 232 and the inside diameter of a recess 236 inpiston 196. The press fit minimizes any motion between the piston 196and the pin 232. The preferred material for the plug 230 is a polymersuch as polyethylene. It is both flexible which makes a good press fitand has a melt temperature very close to the phase change temperature ofthe wax. Polyethylene is also compatible with the surrounding reactorcoolant pump environment. To avoid potential creep, it is important thatplug 230 is encapsulated. If a load is applied to pin 232 in thedirection of the plug 230, without the plug being contained by thepiston 196, the increased force on the plug 230 would cause the plug tocreep radially outward and/or buckle. As configured, with an increasingload on the plug 230, the plug would have to extrude along the thinboundary 234 in order to escape. Due to the nature of the polymer bonds,extrusion through the small gap between the pin 232 and the piston 196is extremely difficult until a higher temperature is obtained. Theholding force from the pin/plug combination can easily exceed 100 lbsprior to activation.

During thermal activation, above normal operating temperature, plug 230becomes soft before the wax begins to change state. As the temperaturefurther increases, the plug can reach melt temperature either prior toor as the piston begins to move. As the plug 230 melts, the plugmaterial becomes viscous and freely flows around pin 232 andsubsequently permits translation of pin 232 within the piston recess 236such that the piston is free to activate the shut down seal.

FIG. 13 shows an alternate embodiment for keeping the piston 196 fromtranslation prior to thermal activation, To prevent the piston 196 frommoving prior to actuation, pin 232 is slidably fixed within bore orrecess 236. The leafs 244 between slots 238 in pin 232 are elasticallybiased outward in the radial direction (like a leaf spring) to keep theoutside diameter of the pin 232 in intimate contact with the insidediameter of the piston bore 196 at the interface 240. Shear pinprojections 242, that extend radially outward from the pin 232 preventthe pin from actuating until the piston 196 exerts sufficient force toshear or break off the projections 242, thereby permitting full travelof the piston 196.

Thus, this improved actuator has a simplified thermal retracting designthat has a higher output force and fewer components than the previousdesign described above. The previous design of the actuator uses HNBRO-rings with the life expectancy which may be less than the desiredtwelve years of operation. The seal arrangement in the preferred designuses long life EPDM O-rings and PEEK seals to provide separate andredundant boundaries for the thermal retracting actuator components.

While specific embodiments of the invention have been described indetail, it will be appreciated by those skilled in the art that variousmodifications and alternatives to those details could be developed inlight of the overall teachings of the disclosure. Accordingly, theparticular embodiments disclosed are meant to be illustrative only andnot limiting as to the scope of the invention which is to be given thefull breadth of the appended claims and any and all equivalents thereof.

What is claimed is:
 1. An actuator comprising: a cylinder having anaxial dimension; a piston axially movable within the cylinder with thecylinder having an upper and lower end which is sealed around thepiston, the piston having one end connectable to an operator; a cavityoccupying a space within the cylinder between the upper and lower endsand which space at least partially surrounds a portion of the piston, anaxial dimension of the piston extending through the space within thecavity when the piston is in a first position of its travel, the axialdimension of the piston having at least two separate diameters with alargest of the diameters leading a smaller of the diameters in adirection of travel of the piston to move the piston to a secondposition, the two diameters occupying at least a portion of the spacewhen the piston is in the first position; a material occupying at leasta portion of the space within the cavity, the material expanding upon anincrease in temperature to exert a force on the piston that causes thepiston to move to the second position when the material rises above apreselected temperature; and wherein the piston extends axially throughthe material occupying a portion of the space within the cavity.
 2. Theactuator of claim 1 including a safety lock configured to prevent thepiston from moving from the first position to the second position whenthe material is substantially below the preselected temperature and freethe piston to move in a direction toward the second position when thematerial rises approximately above the preselected temperature.
 3. Theactuator of claim 2 wherein the lock is thermally activated andconfigured to passively unlock the piston when the fluid is above thepreselected temperature.
 4. The actuator of claim 2 wherein thethermally activated safety lock comprises a pin that is suspended fromone end of the cylinder and extends in a direction that the piston movestoward the second position, the pin extending partially within a recessin an end of the piston, a substantial remainder of the recess beingsubstantially filled with a thermally activated material, wherein thethermally activated material has a viscosity at temperatures below thepreselected temperature that prevents the thermally activated materialfrom flowing alongside a side of the pin and out of the recess and attemperatures substantially at or above the preselected temperature thethermally activated material has a reduced viscosity that enables it toflow alongside the side of the pin and out of the recess enabling thepiston to move in a direction toward the second position.
 5. Theactuator of claim 4 wherein the thermally activated material is apolymer.
 6. The actuator of claim 5 wherein the polymer is polyethylene.7. The actuator of claim 3 wherein the safety lock comprises a shear pinthat is configured to form an obstruction that prevents the piston fromsubstantially moving toward the second position when the material issubstantially below the preselected temperature and when the force onthe piston is large enough to move the piston in a direction toward thesecond position, the shear pin ruptures or otherwise deforms to removethe obstruction.
 8. The actuator of claim 7 wherein the safety lockcomprises a pin that is suspended from one end of the cylinder, in adirection that the piston moves toward the second position, whereinsubstantially below the preselected temperature the pin extendspartially within a recess in an end of the piston, a substantialremainder of the recess being substantially accessible to accommodatepiston travel, the pin having projections in the form of one or moreshear pins or rings which prevent the pin from substantially moving intothe substantial remainder of the recess when the material issubstantially below the preselected temperature, wherein the projectionsare configured to rupture or otherwise deform to enable pin travel intothe substantial remainder of the recess when the force on the piston islarge enough to move the piston in a direction toward the secondposition.
 9. The actuator of claim 1 wherein the force is exerted overan area around a circumference of the piston wherein at least a portionof the at least two separate diameters of the piston extend.
 10. Theactuator of claim 1 including a first seal supported between the cavityand the piston at a lower end of the cavity and a second seal supportedbetween the cavity and the piston at an upper end of the cavity, thefirst and second seals being operable to substantially confine thematerial to the cavity.
 11. The actuator of claim 10 wherein at leastone of the first seal and second seal are cup seals.
 12. The actuator ofclaim 11 wherein the cup seals are constructed out of PEEK.
 13. Theactuator of claim 10 including a backup seal for either or both of thefirst and second seals.
 14. The actuator of claim 13 wherein the backupseal is an o-ring seal.
 15. The actuator of claim 14 wherein the o-ringseal is formed from EPDM or HNBR.
 16. The actuator of claim 10 whereineither a support for the first seal or a support for the second seal isdesigned to relieve a pressure within the cavity when the pressureexceeds a predetermined value.
 17. The actuator of claim 1 including aspring supported within the cylinder and configured to bias the pistonin the first position while the material remains below the preselectedtemperature.
 18. The actuator of claim 17 wherein the force exerted onthe piston is substantially greater than a force exerted by the spring.19. The actuator of claim 1 wherein at least a portion of a wall of thecavity has a reduced thickness that will bulge outward when pressurewithin the cavity exceeds a design value to reduce the pressure withinthe cavity to a safe level.